US 7331027 B2
A method is disclosed for improving design criteria and importantly timing criteria following a metal-only engineering change. The method involves making initial logical changes involving new books (gate-level, filler-cell circuits, called ‘eco books’), running placement and routing with the new books, and timing the resulting logic. If there are timing violations, existing, non-filler books which are in close proximity are considered for swapping with the eco books. The book swaps are all done with wire connections only (i.e. the book placements are not affected). This way, critical paths and non-critical paths can be traded-off to achieve a faster design, even though books are not allowed to be moved. Some simple algorithms are discussed; however, there are many heuristic and analytic algorithms that can be applied in choosing swaps, based on the needs of the particular design.
1. A method performing metal-only engineering changes in a design comprising the steps of:
making the preliminary design changes in a metal-only process to change the existing design and perform an engineering change only (ECO) place and wire change to ensure that only wire changes can be made to the design, and
for timing violations, determining which violating books of the existing design are contributing to said timing violations and creating an incumbent list of violating books which are contributing to timing problems of the design because of one or more of: slowness, poor gains, bad slews, and poor location violations and identifying these violating books as incumbent list books,
determining a replacement book for each violating book by creating a candidate list of replacement books having improved factors because of one or more of: distance, slack, gain and slew from within the existing design, and
swapping incumbent books with said replacement books of the candidate list to make the best swap for each book swapped from within an existing design with only wire changes to the design to improve a path having the said timing violation after comparing slews, gains, slacks and locations of these swapped books with those of the incumbent list and fixing those on the incumbent list by propagating a new critical timing path to swapped candidate books with a metal only fix which does not break the timing of the path involving incumbent books.
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7. A method performing metal-only engineering changes in a design comprising the steps of:
making the preliminary design changes in a metal-only process to change an existing design and perform an engineering change only (ECO) place and wire change to ensure that only wire changes can be made to the design , and
exercising the design, and then
determining if there are violations which occur of the design criteria within the existing design, and
for said design criteria violations, determining which violating books are contributing to said design criteria violations within the existing design and listing those violating books which are contributing to timing problems of the design because of one or more of: slowness, poor gains, bad slews, and poor location violations as incumbent list books in an incumbent list, and
determining a replacement book for each violating book by creating a candidate list from within the existing design of replacement books having improved factors because of one or more of: distance, slack, gain and slew, and
swapping incumbent books with replacement books of the candidate list to make the best swap for each book from within the existing design with only wire changes to the design to improve a path having the said design criteria violation after comparing slews, gains, slacks and locations of these swapped books with those of the incumbent list and fixing those on the incumbent list by propagating a new critical timing path to swapped candidate books with a metal only fix which does not break the timing of the path involving incumbent books.
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IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. and other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies.
1. Field of the Invention
This invention relates to swapping circuits in a metal-only, engineering change only (ECO) process using wire swaps to allow for better slews and timing.
2. Description of Background
This invention relates to an engineering change only (ECO) process where timing and slews can be improved by effectively swapping similar circuits using wiring changes. Typically, in IBM's engineering processes wire-only changes are used to make minor tweaks to a chip design by keeping all the silicon masking while changing the wire masks in the chip fabrication process.
In fact, even logic circuits can be created with metal, provided special ‘filler cells’ are used. These filler cells contain un-customized transistors. By connecting up these transistors in the right way, different logical functions can be created (i.e. NAND3, OR2, Inverter, etc.).
This way, wafers can be created in large quantities up through the wire customization steps. Then, if new problems are detected in the design, the design fixes can be made simply using wire changes that get applied to make new masks to apply to the existing sub-stock. The effect is a quicker chip fabrication from the last detected problem/fix until chip availability.
One problem with metal-only engineering changes is that the metal-customized filler cell gates are generally not as fast as original silicon gates due to the limitation of making the filler cells growable. Also, it is difficult to provide all the flavors of filler-cell books (due to high cost of design and modeling efforts) and there is a likelihood that some of these books would never get used. There may also not be enough contiguous space for some of the larger books required in an engineering change since much of the space may have already been used up.
It would be desirable to have a way to design using filler cells that were just as good as original silicon books due to the cost of modeling efforts. Also, if a new path becomes the most critical path in an area, it would be nice to have the flexibility of re-choosing which circuits get used for which applications to achieve the same function with better performance.
This invention provides a way for making wire-only design changes while maintaining or improving system path performance. In accordance with the preferred embodiment, the engineering changes are made using any current engineering change method. The changes can be fully placed and wired, or simply placed with wire estimates. Then, the design is exercised to determine if there are any design violations. If so, the invention can be invoked. In the preferred embodiment using a design exercised by timing, a determination is made if there are any slew and/or slack timing violations. A replacement candidate is selected and the violation replaced by the processed described in detail below.
Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
The first step of the invention is to use the timing model to classify all books in the design. First, they are classified by pin-compatible function (e.g. NAND3, NOR2, AOI22, etc.) and put on the appropriate list as potential ‘candidates’ for swapping. Next, they are analyzed for original slack (i.e. slack of the worst path each book is involved in), gain, beta ratio (or any other parameter that will be used for book swap selection). These properties are associated with the books (i.e. through an associated list or look-up table).
Then, the timing model is analyzed to determine which books (circuits) are contributing to the timing problems. These could be books that are slow, have poor gains, cause bad slews, have poor locations, etc. These books to be replaced are referred to as ‘incumbent’ books.
The invention then calls for the ‘BEST replacement’ candidate to be chosen for each incumbent. Distance can be factored in as well as slack, gain, and slew. Swaps are made between incumbents and candidates. Then, the model is re-timed.
Results of the preferred embodiment did not require any reiteration of the invention, although that could certainly be done.
In order to understand the invention in more detail, it is important to review the engineering change metal-only (ECO) process. Turning to
There are times when a logical change is needed to enhance a function or fix a design.
The current invention allows for a solution that can improve timing tremendously. In this simple example in
Please note that there are many algorithms that could be chosen to select which books to swap. These swaps could all be determined for the entire design and then all applied at once. The swaps could also be done incrementally (i.e. one at a time) and the design re-timed after each swap. The algorithm could also involve substituting every possible equivalent book in a greedy algorithm and saving the best swap for each case.
The preferred embodiment was implemented with a program (a perl script). The following parameters were used by the program:
In the preferred embodiment, new NAND2 gate, 201 would be the incumbent gate. Suppose its slack is −120 ps and its physical location is x, y (microns from origin). Its slew is 300 ps and its delay is 210 ps. With a technology whose slews should be less than 100 generally, the 300 is extreme. This is a good indication that the book does not have enough gain for the surrounding circuits. Therefore, it would be selected as an incumbent. Suppose NAND2 gate, 106, has a slack of +520, a power level of 8, and is at physical location x+80, y−40. This equates to a distance of 80+40=120 um from the incumbent, new NAND2 gate, 201. Since the distance, 120 um, is less than RADIUS=200 um, the power 8 is greater than MINPOWER=6 and the original slack +520 is greater than PSLACK=200, this is a candidate. Assuming the 120 um is the closest book, this candidate would be the best choice.
This invention allows for further automatic changes to the design. After the slack is determined to be unacceptable, 703, the invention allows the step of creating an incumbent list, 705. This consists of filtering all books out of the design that have poor slews, slack, etc. These are typically the new filler cell gate array books, but could also be any other books in the original design that have timing problems.
The next step is to create a candidate list, 706. This can be accomplished in many ways. The preferred embodiment uses a perl program to create a linked list for all books of each type. Pre-filtering of this list is done for speed (i.e., only books with a power level greater than MINPOWER=6, slack>PSLACK, etc. are added to the table). Each book also has an associate list with properties of placement, slack, slew, gain, book type, etc. For each incumbent, the book type is extracted. The linked list for that type book is traversed. For each book, the distance is calculated and any additional filtering could be done as well (i.e. slack, slew). All books that qualify are added to a table of possible substitutions, but the closest book is also added to a FINAL list of swaps. The program can output JUST the FINAL swaps or the list of all possible swaps.
The next step is the make the best swap for each book, 707. The preferred embodiment is a perl script which runs outside the timing environment. It is fast and will determine all the swaps in less than a minute. However, it is also possible to have the program run in an incremental timing environment and determine just the first swap. The swap can then be made, the results analyzed, and the program come up with the next swap. Care has to be taken to keep the program from looping on the same group of swaps.
Once the swaps are made, 707, timing is run again, 702, and the slacks reanalyzed, 703. If the slacks are acceptable, the design is done, 704. If not, the process can be iterated again with different parameters.
While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. This invention could also be applied to fixing other types of problems (i.e. non-timing) in a metal-only EC. For instance, if testability or fault tolerance need improvements, violations detected for these criteria could corrected by replacements applied as additional features and advantages are realized through the techniques of the present invention.
These claims should be construed to maintain the proper protection for the invention first described.